March 13, 2013

Neanderthals Went Extinct Because Of Their Large Eyes

Scientists know that early Humans (Homo sapiens) and Neanderthals (H. s. neanderthalensis) coexisted for a short time before the latter eventually became extinct. While it was understood that humans had better developed brains than their more primitive counterparts, it was generally not well-known why these early ancestors made a grand exit.

A new study, published in the journal Proceedings of the Royal Society B, suggests one reason the Neanderthals went by the wayside was because of their large eyes, which were much larger than those of our own species. As a result of these large oculars, more of their brain was devoted to seeing in the long, dark nights than for high-level processing.

Without needing to spend so much time helping our eyes see, ourbrain was able to help our ancestors learn to fashion warm clothes and develop social structures to survive the impending Ice Age. While Neanderthals had the same size brain as our early ancestors, theirs were organized much differently and focused not just on vision, but also on control of the body, which may have played a part in their demise.

So the long-lived debate over why Neanderthals suddenly vanished from the Earth about 28,000 years ago may finally be solved.

For the study, Professor Chris Stringer of the Natural History Museum, along with Eiluned Pearce and Robin Dunbar of the University of Oxford, examined the skulls of 25 Neanderthals and 39 modern humans, all between 27,000 and 200,000 years old.

The team analyzed measurements of the inside volume of the skulls, eye socket widths, breadths and volumes, allowing the team to get a good estimate of the size of the eyeballs and visual cortices (brain areas associated with vision). The measurements confirmed that H. s. neanderthalensis eyes were much larger — on average a quarter-inch larger than those in H. sapiens.

Although this seems rather insignificant, Pearce noted that it was enough to ensure that Neanderthal brains were working harder on processing visual information than on other more important areas.

"Since Neanderthals evolved at higher latitudes, more of the Neanderthal brain would have been dedicated to vision and body control, leaving less brain to deal with other functions like social networking," she told BBC News.

Living in higher latitudes meant less light in a 24-hour period, possibly explaining why Neanderthals in the northern areas had evolved larger eyes. For comparison, Pearce noted that modern humans living in the higher latitudes also have larger eyeballs and visual cortices than their more equatorial peers.

Previous research has shown that Neanderthals also had bigger bodies than modern humans and as such their brains had to focus more on body maintenance and control.

Since the Neanderthal brain didn´t grow to compensate for the larger eyes and visual cortices, the team suggests their brain regions were organized much differently than our own. Neanderthal noggins likely didn´t have the appropriate accommodations to host large-scale social structure or increased cognition, according to the team.

These lacks of function could have limited their ability to cope with environmental change and competition from the more advanced early H. sapiens, added the study authors. In the end, this would have ultimately led to their extinction, especially if no other factors were at play.

“Thelarge brains of Neanderthals have been a source of debate from the time of the first fossil discoveries of this group, but getting any real idea of the ℠quality´ of their brains has been very problematic. Hence discussion has centered on their material culture and reconstructed way of life as indirect signs of the level of complexity of their brains in comparison with ours,” said Stringer in a statement.

“Recently, their possibly accelerated life history, with shorter childhoods, has been brought into the discussions, as have possible differences in brain physiology deriving from research on their DNA,” he added.

Apart from their inability to from large social groups and to fashion warm clothes for survival, the research is not trying to paint Neanderthals as a stupid race.

Dunbar, who was also part of other emerging research portraying the early relatives of H. sapiens as intelligent beings, said the new study made sure to not lean toward a stereotypical image of Neanderthals, one that is often portrayed in Hollywood as them being stupid and brutish creatures.

"They were very, very smart, but not quite in the same league as Homo sapiens," he told BBC News. "That difference might have been enough to tip the balance when things were beginning to get tough at the end of the last ice age.”

Until now, most of the knowledge of the Neanderthal brain was based on skull casts. These past studies have offered a good indication on the size and structure of the brain, but have done little to indicate how the actual brain may have functioned. The latest study is an imaginative approach in trying to address this issue.

The new study concurs with other research that has shown a larger eye size in primates is proportional to the amount of brain space devoted to visual processing. That research led Stringer and his colleagues to assume the same would be true of Neanderthals.

“Our study provides a more direct approach by estimating how much of [the Neanderthal] brain was allocated to cognitive functions, including the regulation of social group size; a smaller size for the latter would have had implications for their level of social complexity and their ability to create, conserve and build on innovations,” concluded the authors.